1. Field of the Invention
The present invention relates to a system for compensating for a pitch error of a ball screw in a numerically controlled machine tool, and more particularly to a pitch error compensating system suitable for precision machining.
2. Description of Related Art
Ball screws for controlling movable components of numerically controlled machine tools do not have accurately constant pitches. Pitch errors of such ball screws are compensated for by computerized numerical control apparatus to increase positioning accuracy. According to a general pitch error compensating system, the strokes of respective axes are divided into suitable periods, and pitch error corrective quantities are measured in the respective periods. The measured corrective quantities are then stored in a nonvolatile memory. Each time an axis moves, the pitch error corrective quantity that corresponds to the present value of the axis is read from the memory, and added to or subtracted from interpolated pulses, thereby compensating the pitch error.
FIG. 6 of the accompanying drawings shows, by way of example, pitch error corrective pulses in a conventional pitch error compensating system. In FIG. 6, a first period P.sub.0 -P.sub.1 has a pitch error corrective quantity .epsilon.1, a second period P.sub.1 -P.sub.2 a pitch error corrective quantity .epsilon.2, and a third period P.sub.2 -P.sub.3 has a pitch error corrective quantity .epsilon.3, each period being indicated as a pitch error correcting period l. The pitch error corrective quantity in each period is outputted as a pitch error corrective pulse at an intermediate point of the period, and added to interpolated pulses.
A pitch error corrective quantity is outputted at once as a pitch error corrective pulse. If the pitch error corrective quantity is large, then the corresponding error corrective pulse is also large. The large error corrective pulse that is outputted at once is not preferable from the standpoint of positioning accuracy. Such pitch error corrective pulse may lower the quality of the machined surface in precision machining.